Generally, a liquid crystal display (LCD) device gradually expands its application area thanks to its characteristics of lightweight, a slim profile, and low power consumption. With this trend, the LCD is used for office automation apparatus and audio/video apparatus.
The LCD device displays a desired image on a screen by controlling a light transmittance and an image signals applied to a plurality of switching devices arranged in a matrix type.
Since the LCD device is not a self-emission type display device, the LCD device requires a light source such as a backlight. A cold cathode fluorescent lamp (CCFL) is used as the light source used in the backlight.
The CCFL uses electron emission phenomenon caused by applying a strong electric field on a surface of a cold cathode, and it is suitable for realizing an LCD device with low heat generation, high brightness, long lifetime, full color, etc. The CCFL is classified into a light guide type, a direct light type, a reflective plate type, and so forth, and an appropriate typed one is employed depending on an LCD device. Ne, Ar, Hg, etc is used as a gas filled inside the CCFL.
As a lamp driving method used in the LCD device, two methods are being widely used, of which one is a method in which one lamp is driven by one transformer, and the other is a method in which a plurality of lamps are driven by one transformer.
When using a plurality of lamps, the temperature of the lamp increases as the operational time of the lamp increases. In this case, temperature gradients of lamps disposed at an upper portion are different from those disposed at a lower portion, which leads to temperature difference among the lamps. Accordingly, an impedance of the lamp changes so that the magnitude of current flowing through each of the lamps becomes different. That is, as the temperature of the lamp increases, gases filled inside the lamps actively move. This causes the impedance of the lamp to be lowered, and the current flowing into the lamps to be increased.
Consequently, a difference among the currents flowing through the lamps occurs inevitably, that is, there is current offset, causing the brightness of light emitted from the entire lamps to be non-uniform.